GMC SIERRA 1994 Manual page 223

I
NOTICE:
In cold weather, water can freeze and crack the engine, radiator,
heater core and other parts. Use the recommended coolant.
When the coolant in the coolant recovery tank is at or above the
COLD
mark, start your vehicle.
If the overheat warning continues, there's one more thing you can try. You
can add the proper coolant mix directly to the radiator!, but be sure the
cooling system is cool before you do it.
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